Robin Ultralight: Engine cowl part 2

Welcome to my Robin Blog.

It was suggested to me that I start a Blog on my ultralight project the "Robin". I have been working on this project for 4 years. On one of my first days at Vought aircraft, a stress man and future friend named Kenny Andersen walked up to me and said, "Aren't you the Mark Calder that designed the Wren Ultralight" Why yes I am I said. "well what have you done lately?" That was the genesis of the Robin design. The first 2.5 have been spent in the design phase. Actual construction started 1.5 years ago and has actually progressed smoothly. There have been a number of changes from the onset, but for the most part it is following my original concept. I will eventually sell plans for the Robin and make available all molded parts, fittings and welded assemblies. The Robin is designed to FAA part 103 and as such requires no pilots license to fly, although I think its a good idea to actually learn how to fly!! The actual name "Robin" was my Daughter Jamie's idea, I asked her to name the design based on my "cute little bird" theme (Wren)

Every good aircraft design has a "Mission" in mind before the actual design is started. A good designer will refer back to this mission every time a design decision must be made. Good design after all is just a series of good design decisions. On my first Ultralight design the Wren, the mission was to design a high performance low powered aircraft. The reduction of drag was the prime concern. I had been flying powered Hang gliders prior to this and because of this experience, I placed a high priority on climb performance. While most designers chose bigger engines, I chose lower drag and high aspect ratio (low span loading) wings. The Wren could out climb conventional Ultralight with up to 65 hp. The Robin follows this philosophy, but tries to improve on the performance of the Wren. Ultralight are not built by "rich" people, they offer an inexpensive means to enjoy one of the greatest experiences of my life, low speed soaring and flying.

Design Concept

The cost of an aircraft is directly proportional to its weight. , if low drag can be achieved then lighter and cheaper engines can be used. The Robin expands on the design mission of the Wren by using a longer span (40') wing and using a low speed laminar flow airfoil, (Wortmann FX 170) The leading edge of the wing on the prototype is molded fiber glass. The spar has been placed at 33% of the wing chord because the chosen airfoil is laminar over the first 32%. The aft covering is light weight Dacron Fabric. The leading edge of this fabric is purposely pinked and placed at the 32% chord point to facilitate laminar transition and elimination of separation bubbles. The main difference between the original design of the Robin and the current final design is the elimination of the single mono wheel retractable landing gear. Part 103 does not allow for a retractable landing gear. Which is really unfortunate because I spent a long time designing a really neat mechanism!!

In the course of the 4 years I have worked on the Robin, the structural design concept has evolved radically. Originally I was going to draw on the design of the Wren and use essential the same construction concepts. The original design of the Wren was heavily influenced by my Friend Steve Wood's Sky Pup design. I lived in Wichita Kansas and worked at Cessna Aircraft along with Steve. I watched his progress on the Pup and was very impressed with his concepts. I adapted the concept of using Styrofoam sheeting as the shear panels for the fuselage and the wing ribs. I did not however use the foam for the shear webs of the wing as Steve did. I originally wanted to build the fuselage of the Robin in a similar manner. Weight and the desire to not use foam for the basic structure due to the danger of fuel leaking eventually drove me to a all wood fuselage design. The wings were designed to take advantage of the Graphlite carbon pultruded material pioneered for the experimental aircraft by Jim Marske. I was familiar with this product from my experience at Bell Helicopter where it was considered in the construction of the V-22 wing.

lower Foam blocked in place

The lower half of the foam shaping started by flipping over the fuselage.

I want to thank my Friend and neighbor Ed Gardner for all of his help on this one. He's a pretty big boy, airplane nut and an all around good guy!!
check out Eds project and Business: http://mmwauto.com/index.php/services.html

As shown in the upper cowl blog, the foam was fitted between the firewall and the 1/4" foam space that was attached to the rear of the spinner mold.

Again the rules for attaching the foam is to make sure the glue is well below the final contour and that internal cut outs are also below the final contour. Its not a disaster if they are, as you will see later, I ended up with a small hole.

Another view of the blocked in foam

After the foam is blocked in, I started the initial shaping. rough cuts were made with hand saw and final cuts were made using a sanding foam block with 36 grit paper. I have made other cowls before using higher density urethane foam, the result is similar, but the higher density just takes a lot longer to shape.

rough shaping with a hand saw.

I first hit on this idea of using floral foam when i built a cowl for an abandoned project you sometimes see hanging on my wall. I used to build a number of sail plane models and the nose cone was always carved out of a block of Balsa. The carving technique was always to profile the plan view and then the side view. after that the rest was sanded away. I found out about Telescope hobbyists using glass to grind glass and I realized that floral urethane foam would do the same. This really works quite well.

More rough cutting

You will notice in this picture that the center line template was exposed by sanding. I use 36 grit paper when I do this. There is no need to use any finer, it only slows down the process. The side templates will also be sanded to profile the plan view.

Final shaping

After the rough cuts were made, scrap pieces of the foam were used to finish the final sanding. as you can see there is a small hole in the foam, I'm not worried about this because the next step is to lay a layer of fiberglass and eventually fill and fair the surface with Bondo or Automotive body filler.

Another view of the final shape

I continued to 'tweek" and fine tune the shape for a little while after this. Once the basic shape of the main cowl is finished, I will start the process of adding the cheek cowls. These cowls will be assymetrical, The R/H cowl will be the fairing for the exhaust and the cooling air outlet. The L/H cowl will have an air inlet and also fair the carb and air filter.

Side profile before fiberglass is applied

Before fiberglass is added, the surface is prepped with a mixture of resin and Micro Balloon. The Micro is added to about 4 OZ of resin and mixed to the consistency of pancake batter. The purpose of this is to fill the open cells of the foam. If you do not fill the cells, the laminating resin will eventually wick into the cells and draw air into the laminate.

One layer of 8 OZ cloth added

Here is a picture of the cloth overlaid on the foam. I use the 8 oz Rutan style BID cloth. This is a 8 harness crowfoot weave style cloth. It is extremely drapable and will conform to almost any contour. I added layers of aluminum tape so I can have an excessive flange. This will be important later in the mold process. The tape on the side will be in the location of the eventual cheek cowl. A couple of plies of Peel ply were also laid at the lower edge of this lay up. They are removed before the upper surface is laid up. They allow a smooth transition and a bond able surface when peeled off.

Initial coats of Bondo applied

The process was repeated for the upper surface and the first layers of Bondo were added. The idea on the bondo is to completely coat the fiberglass to hide and of the weave and to smooth out any small waves in the surface. I will not completely finish out the master at this stage, I still have to add the cheek cowls.

View looking aft

This is a good view of the upper and lower surfaces. I didn't have to spend a great deal of time finishing the sides in the areas of the cheek cowls.

Ready for the Cheek Cowls.

The next step is preparation for the cheek cowls. I'm going to try something I have never done on this step. I will build up the cowls on the bench similar to the way I have done my wing tips. I will then remove the cowls from the backing board. I plan on temporally gluing some sand paper to the fuselage and then sand the contour into the cheek cowls.

That's the next blog entry.

Welcome to my Robin Blog.

Engine cowl part 2

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